Nuclear power plant

Nuclear power plant consists of the following elements. These are

Nuclear fuel

Fuel of a nuclear reactor should be fissionable material which might be defined as an element or isotope whose nuclei are often caused to endure nuclear fission and to produce a fission chain reaction. It will be one or all of the following

U233, U235 and Pu239.

Natural uranium found in earth crust contains three isotopes namely U234, U235 and U238 and their average percentage is as follows

U238 — 99.3%,U235 — 0.7%,U234

Moderators

In any chain reaction, the neutrons produced are fast moving neutrons. These are less effective in causing fission of U235 and try to escape from the reactor.
The speed of these neutrons must be reduced if their effectiveness is carrying out fission is to be increased.
This is done by making these neutrons collide with lighter nuclei of other materials that will not absorb these neutrons however simply scatter them.
Each collision causes loss of energy and so the speed of neutrons is reduced. Such a material is called a ‘Moderator’.
The neutrons slowed down are easily captured by the fuel element at the chain reaction proceeds slowly.
A moderator should process the following properties
- It should have high thermal conductivity.
- It should be available in large quantities in pure form.
- It should have high melting point in case of solid moderators and low melting point in case of liquid moderators.
- It should provide good resistance to corrosion.
- It should be able to slow down neutrons

Reflectors

During fission time, the produced neutrons will be partly absorbed by the fuel elements, moderator, coolant and other materials.
The remaining neutrons will try to escape from the reactor and can be lost. These losses are reduced by surrounding the reactor core with a material called a reflector which will reflect the neutrons back to the core. They improve the neutron economy.

Shielding

During Nuclear fission alpha, beta, comma particles and neutrons are also produced.
They are harmful to human life.It is necessary to shield the reactor with thick layers of lead or concrete to protect both the operating personnel as well as environment from radiation hazards.

Cladding

In order to prevent the contamination of the coolant by fission products, the fuel element is covered with a protective coating. This is known as cladding.
Control rods are used to control the reaction to prevent it from becoming violent.
They control the reaction by absorbing neutrons. These rods are made of boron or cadmium.

Coolant

The main purpose of the coolant in the reactor is to transfer the heat produced inside the reactor. The same heat carried by the coolant is used in the heat exchanger for further utilization in the power generation.
Some of the desirable properties of good coolant are listed below
- It must not absorb the neutrons.
- It must be non-corrosive.
- It must have high boiling point (if liquid) and low melting point (if solid)
- It must be non-oxidizing and non-toxic.

Nuclear reactor

A nuclear reactor may be regarded as a substitute for the boiler fire box of a steam power plant.
Heat is produced in the reactor due to nuclear fission of the fuel U235.
The heat liberated in the reactor is taken up by the coolant circulating through the core.
Hot coolant leaves the reactor at top and flows into the steam generator (boiler).

Radiation hazards

The reactor is a source of intense radioactivity. These radiations are very harmful to human life.
It requires strong control to ensure that this radioactivity is not released into the atmosphere to avoid atmospheric pollution.
A thick concrete shielding and a pressure vessel are provided to prevent the escape of these radiations to atmosphere.

Steam generators

The steam generator is fed with feed water which is converted into steam by the heat of the hot coolant.
The purpose of the coolant is to transfer the heat generated in the reactor core and use it for steam generation.
Ordinary water or heavy water is a common coolant.

Turbine

The steam produced in the steam generator is passed to the turbine and work is done by the expansion of steam in the turbine.

Coolant pump and feed pump

The steam from the turbine flows to the condenser where cooling water is circulated. Coolant pump and feed pump are provided to maintain the flow of coolant and feed water respectively.

Advantages of nuclear power plant

It can be easily adopted where water and coal resources are not available.
The nuclear power plant requires very small quantity of fuel. Hence fuel transportation cost is less.
Space requirement is less compared to other power plants of equal capacity.
It is not affected by adverse weather conditions.
Fuel storage facilities are not needed as in the case of the thermal power plant.

Disadvantages of nuclear power plant

Radioactive wastes if not disposed of carefully have adverse effect on the health of workmen and the population surrounding the plant.
It is not suitable for varying load condition.
It requires well-trained personnel.
It requires high initial cost compared to hydro or thermal power plants.

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